Here is my reasoning. Im sure it isnt the consensus of the Fathers or whatnot, but its MY consensus.
If you look at the Biblical creation myth, whether you hold it to be entirely true physically or not, there are many spiritual truths to this illustration.
God created one man and one woman, and intended them to be united in the bonds of marriage, just as the Trinity is united eternally in Divine Love. God ordered the man and the woman to multiply (i.e., get it on). THIS is the formula (or "context" as Pikhristos was talking about) that God clearly intended for sex. I dont know if there is any interpretation of this anywhere else, but the way I see it is that sexual intercourse outside of this "context" is sin.
The Bible often uses the word "porneia" as something that is forbidden, which means an illicit sexual act. Erasmus and most other translators of the Bible into English translate the word as "fornication", which means sex outside of marriage. If you trust that the Holy Spirit guided these translators to provide an accurate English translation of the Bible, then the question ends there. Sex outside of marriage is forbidden.
But, if you still think that fornication is too narrow of a translation for porneia, and think that it should not refer to sex outside of marriage, then it is important to look at other extrabiblical early Christian writings to see how the early Christians interpreted the Bible. Everyone sees the Bible through their own set of glasses. It is important to see the Bible through the glasses of the early Church so that one can have a proper understanding of it. Let's look at what some early Christian texts say about sex outside of marriage.
“ you shall not commit adultery; you shall not corrupt boys; you shall not be sexually promiscuous; you shall not steal; you shall not practice magic; you shall not engage in sorcery; you shall not abort a child or commit infanticide. (Did 2:1-2)"
Would not "sexually promiscuous" refer to sex outside of marriage?
It is also clear that abortion was condemned by the Early Church. It is also condemned by some of the Jewish pseudoepigrapha. Lets take a look at Enoch (the Ethiopians consider this to be Scripture).
“revealed to the children of the people (the various) flagellations of all evil,...the smashing of the embryo in the womb so that it may be crushed...” (1En 69:12)
“In those days, the nations shall be confounded, and the families of the nations shall rise in the day of the destruction of the sinners. In those days, they (the women) shall become pregnant, but they (the sinners) shall come out and abort their infants and cast them out from their midst; they shall (also) abandon their (other) children, casting their infants out while they are still suckling. They shall neither return to them (their babes) nor have compassion upon their beloved ones.” (1En 99:4-5)
Earlier you said abortion wasnt bad. Well, it looks like the early Christians and even the ancient Jews would have disagreed with you there.
Much later, St Augustine (highly revered, in the Western Church especially, for his wisdom) said
"For necessary sexual intercourse for begetting [children] is alone worthy of marriage. But that which goes beyond this necessity no longer follows reason but lust. And yet it pertains to the character of marriage . . . to yield it to the partner lest by fornication the other sin damnably [through adultery]."
You can see that in the early centuries, the Catholic Church (which according to the Bible is the "pillar of truth"), as guided by the Holy Spirit, did not approve of premarital sex.